The way it tracks so many 'sims', shadows, and lights is why the size is so small, and it is a medium city size from the old games, if they weren't perfect squares then it wouldn't seem so small - they just need to change the border shapes and most people would be happy and not notice the restrictive size...

The size of a region should be based on the usable space, not just a big square that ignores water and mountains

Cities XL is a good game, but it suffers from memory leaks that can't be fixed since the dev went belly up. Compared with SimCity though it's a good game, less restrictive, more open, and offline. Bear in mind though that Cities XL was originally suposed to be an online game